Perforation of wells is a commonly utilized technique for harvesting fossil fuels such as oil and natural gas that are entrained in rock and other geological formations. Wells are drilled into the ground to a sufficient depth to penetrate the rock bed where fossil fuels are located in subterranean formations, typically about one mile or more down. The angle of the well can be turned so that further well drilling can continue parallel to the ground surface for several more feet or miles. The resulting wellbore is fitted with a casing or tubulars to prevent the well from collapsing in on itself during the remainder of the process. A perforating gun is then lowered down the wellbore, such as on a wire, to terminal end or “toe” of the wellbore. A plug at the leading edge of the perforating gun plugs the wellbore isolating each stage of hydraulic fracturing completion. Explosive charges within the perforating gun are then detonated as the perforating gun is retracted a distance up the wellbore. Detonation of the charges creates high pressure, high velocity perforation jet from each charge that blows targeted holes in the sides of the perforating gun and well tubular(s), piercing tight, controlled perforation tunnels through the tubular(s) and cement then into the surrounding rock formation. Once the well or stage is perforated, the perforating gun is fully retracted from the wellbore, proppant, a mixture of sand, water and other fluids, is then sent down the wellbore, passing through each of the perforations and into the surrounding rock. The sand and fluid create a permeable path for natural gas and oil from the rock to enter the wellbore. When all the existing perforation tunnels have been hydraulically fractured within a stage, hydraulic fracturing of that stage ends and the next stage of work begins. Another gun string consisting of a plug and multiple guns is lowered again into the wellbore, and the process of plug isolating, perforating, and fracturing begins on a new section. This process is repeated iteratively until the entire wellbore is exhausted.
Perforating guns include a plurality of charges to create the controlled explosions and quantity of perforation tunnels necessary to perforate the rock formations. However, creating explosions of sufficient force to perforate the gun housing and well casing, while simultaneously also being sufficiently directed and targeted to create a narrow tunnel in the rock formation that permits further fracturing and release of the entrained oil and natural gas without collapsing in on itself, is a feat of engineering. Accordingly, the number, placement and pattern of perforating charges within each perforating gun are critical to optimize harvesting capacity without compromising structural integrity of the tubulars. To further complicate the process, not all stages perform uniformly or provide the same amount of harvested material. Some stages or perforation clusters within a well have more or less productivity as a result of the process, and some perforations or perforation clusters become filled with the proppant faster than others. Therefore, maximizing the number of stages with the highest levels of proppant placed is one way to maximize the total productivity of a well.
There are also limits on the number of stages that can be made based on the limitations of the perforating gun. For instance, current perforating guns used in the majority of wells in the United States have an outer diameter of 2.75, 3.125 or 3.375 inches. Shaped charges each containing 15 to 25 grams of explosive are loaded in the perforating guns. Due to current design of the shaped charges and their orientation within the perforation gun, the maximum density of charges is 6 shots per foot of loaded gun. This corresponds to perforations approximately every 2 inches along the gun barrel length. Guns commonly contain 6 to 18 shots, thus corresponding to 12 to 36 inches of loaded gun barrel where perforations can occur. The shaped charges are arranged either linearly or helically in particular patterns to provide the desired number and placement of perforation tunnels. Each shaped charge, however, takes almost the full inner diameter of the perforating gun barrel to include sufficient amounts of explosive material and the other necessary components, including a metal liner, casing or housing, primer and detonation cord. The multiple shaped charges can be linked together through a common detonation cord, so that all the shaped charges can be detonated simultaneously. Typical perforating guns and shaped charges can punch holes or “perforations” of about 0.23 to 0.72 inches in diameter, and create perforation tunnels of 6 to 48 inches in length.
The size of perforating gun is also limited by the angle of the wellbore, where the wellbore transitions from the vertical direction to the horizontal direction. The perforating gun cannot be so long that it cannot navigate the turn of the well from vertical to horizontal, and cannot be so wide that it spans the entire diameter of the wellbore. In addition, once the wellbore is drilled and the casing established, it takes about 2 hours to send a perforating gun downhole in the wellbore and perforate, with another 2-3 hours to fracture the stage or zone of interest. Therefore, time and money can also play a factor in the efficiency of a wellbore.
Individual charges and shaped charges are well-known in the field of explosive charges used in perforating guns, as are carrier assemblies for loading a plurality of charges within a perforating gun. For example, U.S. Pat. No. 4,800,815 issued to Appledorn describes a carrier assembly having thin walls and a deformable opening for receiving a shaped charge when the shaped charge is inserted through the opening, and subsequently retaining the shaped charge within the opening once placed. It discloses multiple charges of a reduced size at the same lateral location within a perforation gun. However, these are individual, separate charges having separate casings, arranged to abut at their interior ends.
U.S. Pat. No. 7,913,758 to Wheller et al. and U.S. Pat. No. 8,904,935 to Brown both teach the use of multiple perforating or cutting jets arranged so the jets converge at a point. However, in each case, the convergence point is of the perforating jets and is located outside of the shaped charges generating the perforating jets. They therefore do not teach a way to maximize or increase the explosive power of the shaped charges themselves.
There is still a need in the field of well perforating to increase well production overall, such as by increasing the efficiency of well creation.